Question

The following code (condensed from a larger program) does not compile with clang or gcc.

struct S1 {
  void m1() {}
};

template<typename B> struct S2 : B {
  void m2() {}
  void m3();
};

template<typename S, void (S::*m)()> void f1(S* o) {
  (o->*m)();
}

template<typename B> void S2<B>::m3() {
  f1<S2, &S2::m1>(this);
}

int main() {
  void (S2<S1>::*m)() = &S2<S1>::m1;
  S2<S1> o;
  o.m3();
}

Here is clang’s error message:

bad.cc:15:3: error: no matching function for call to 'f1'
  f1<S2, &S2::m1>(this);
  ^~~~~~~~~~~~~~~
bad.cc:21:5: note: in instantiation of member function 'S2<S1>::m3' requested
      here
  o.m3();
    ^
bad.cc:10:43: note: candidate template ignored: invalid explicitly-specified
      argument for template parameter 'm'
template<typename S, void (S::*m)()> void f1(S* o) {
                                          ^
1 error generated.

This code compiles when I replace m1 by m2. Clearly the compiler knows about m1 (different message when I replace m1 by m4), so why should a pointer to it be invalid in this context?

Answer 1

The thing is, the type of m1 is void(S1::*)(void), not void(S2::*)(void). So fix it by leveraging the known base class name:

struct S1 {
    void m1() {}
};

template<typename B> struct S2 : B {
    void m2() {}
    void m3();
};

template<typename S, typename B, void (B::*m)(void)> void f1(S* o) {
    (o->*m)();
}

template<typename B> void S2<B>::m3() {
    f1<S2, B, &B::m1>(this);
}

int main() {
    S2<S1> o;
    o.m3();
}

Of course this doesn’t (yet) scale to methods defined in indirect base classes, but with a bit of TMP it can be done (will see if I can post that while the intermission of Going Native 2012 lasts :))

The more ‘flexible’ approach would be:

template<typename B, typename MF> void f1(B* o, MF mfp) {
    (o->*mfp)();
}

template<typename B> void S2<B>::m3() {
    f1(this, &B::m1);
}

You could/should use typetraits to ensure that S2<B>& is convertible to a B& if the class layout doesn’t already explicitly guarantee that, as in your current example.